U.S. patent application number 11/912953 was filed with the patent office on 2009-03-12 for rolling device employing lubricating grease composition and electric power steering apparatus employing the rolling device.
Invention is credited to Toshihiro Asakura, Hirofumi Kuwabara, Shigeyuki Sugimori.
Application Number | 20090065287 11/912953 |
Document ID | / |
Family ID | 37308006 |
Filed Date | 2009-03-12 |
United States Patent
Application |
20090065287 |
Kind Code |
A1 |
Kuwabara; Hirofumi ; et
al. |
March 12, 2009 |
ROLLING DEVICE EMPLOYING LUBRICATING GREASE COMPOSITION AND
ELECTRIC POWER STEERING APPARATUS EMPLOYING THE ROLLING DEVICE
Abstract
A rolling device is provided which employs a lubricating grease
composition which considerably diminishes irregular friction
fluctuations occurring on rolling/sliding friction surfaces, shows
stable low-torque characteristics in a wide temperature range, can
retain a sufficient oil film even at high temperatures, has along
life, and contains a novel thickener. Also provided is an electric
power steering apparatus employing the rolling device. The rolling
device is characterized by employing a lubricating grease
composition comprising: a thickener which is a mixed thickener
comprising a mixture of (a) one or more diurea compounds which are
one or more aliphatic diurea compounds having an average molecular
weight of 500-750 in which 10-70% by mole of the linear hydrocarbon
groups are unsaturated components, the aliphatic diurea compounds
having been produced using as a raw material one or more primary
amines having a total amine value in the range of 200-400, (b) at
least one fatty acid metal salt, and (c) at least one amide
compound selected from the group consisting of aliphatic amides and
aliphatic bisamides represented by formulae (I) and (II):
R.sub.1CONH.sub.2 (I) R.sub.1CONHR.sub.2NHCOR.sub.1 (II) (wherein
R.sub.1 represents a saturated or unsaturated alkyl group having
15-17 carbon atoms and R.sub.2 represents a methylene group or an
ethylene group), the proportions of (a) and of (b) and (c)
satisfying the relationship: a/(b+c)=0.2-10 provided that (1) the
proportion of ingredient (a) is 1-10, (2) the proportion of
ingredient (b) is 0.5-2.5, and (3) the proportion of ingredient (c)
is 0.5-2.5, the amount of the thickener being 2-30% by weight,
based on the whole composition; (d) a base oil which is a lube base
oil having a pour point of -25.degree. C. or lower and comprising a
synthetic hydrocarbon oil as the main component; and (e) an
additive which is a mixture comprising an organomolybdenum complex,
an organozinc compound of a dithiocarbamic acid, and an organozinc
compound of dithiophosphoric acid, the amount of the additive being
1-7% by weight, based on the whole composition.
Inventors: |
Kuwabara; Hirofumi; (Osaka,
JP) ; Sugimori; Shigeyuki; (Osaka, JP) ;
Asakura; Toshihiro; (Osaka, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
37308006 |
Appl. No.: |
11/912953 |
Filed: |
April 27, 2006 |
PCT Filed: |
April 27, 2006 |
PCT NO: |
PCT/JP2006/308867 |
371 Date: |
October 29, 2007 |
Current U.S.
Class: |
180/444 ;
508/168 |
Current CPC
Class: |
C10M 2215/0813 20130101;
C10N 2040/14 20130101; C10M 2207/1265 20130101; C10M 2205/0265
20130101; C10M 2219/068 20130101; C10M 2227/09 20130101; C10N
2010/06 20130101; C10N 2010/04 20130101; C10M 169/00 20130101; C10N
2010/02 20130101; C10M 2215/1026 20130101; F16C 33/6633 20130101;
C10N 2010/12 20130101; C10N 2050/10 20130101; C10M 2205/0285
20130101; C10M 2207/1285 20130101; C10M 2203/1006 20130101; C10N
2030/08 20130101 |
Class at
Publication: |
180/444 ;
508/168 |
International
Class: |
B62D 5/04 20060101
B62D005/04; C10M 169/06 20060101 C10M169/06 |
Claims
1. A rolling device characterized by employing a lubricating grease
composition comprising: a thickener which is a mixed thickener
comprising a mixture of: (a) one or more diurea compounds which are
one or more aliphatic diurea compounds having an average molecular
weight of 500-750 in which 10-70% by mole of the linear hydrocarbon
groups are unsaturated components, the aliphatic diurea compounds
having been produced using as a raw material one or more primary
amines having a total amine value in the range of 200-400, (b) at
least one fatty acid metal salt, and (c) at least one amide
compound selected from the group consisting of aliphatic amides and
aliphatic bisamides represented by formulae (I) and (II):
R.sub.1CONH.sub.2 (I) R.sub.1CONHR.sub.2NHCOR.sub.1 (II) (wherein
R.sub.1 represents a saturated or unsaturated alkyl group having
15-17 carbon atoms and R.sub.2 represents a methylene group or an
ethylene group), the proportions of (a) and of (b) and (c)
satisfying the relationship: a/(b+c)=0.2-10 provided that (1) the
proportion of ingredient (a) is 1-10, (2) the proportion of
ingredient (b) is 0.5-2.5, and (3) the proportion of ingredient (c)
is 0.5-2.5, the amount of the thickener being 2-30% by weight,
based on the whole composition; (d) a base oil which is a lube base
oil having a pour point of -25.degree. C. or lower and comprising a
synthetic hydrocarbon oil as the main component; and (e) an
additive which is a mixture comprising an organomolybdenum complex,
an organozinc compound of a dithiocarbamic acid, and an organozinc
compound of dithiophosphoric acid, the amount of the additive being
1-7% by weight, based on the whole composition.
2. The rolling device according to claim 1, characterized in that
the rolling device is a ball screw mechanism.
3. The rolling device according to claim 1, characterized in that
the rolling device is a rolling bearing.
4. The rolling device according to claim 3, characterized in that
the rolling bearing is a ball bearing which utilizes the rolling of
balls.
5. An electric power steering apparatus characterized by employing
the rolling device of any one of claims 1 to 4.
Description
TECHNICAL FIELD
[0001] The present invention relates to a rolling device employing
a lubricating grease composition which considerably diminishes
irregular friction fluctuations occurring on rolling/sliding
friction surfaces, shows stable low-torque characteristics in a
wide temperature range, can retain a sufficient oil film even at
high temperatures, has a long life, and contains a novel thickener.
The invention further relates to an electric power steering
apparatus employing the rolling device.
BACKGROUND ART
[0002] In recent years, the automobile industry is growing
remarkably and the desired values of performances of individual
automotive parts separately constituted including greases increase
steadily year by year. To provide a new technique is always
required.
[0003] Especially in electric power steering apparatus for motor
vehicles, technical innovations are remarkable. The apparatus,
which at first were employed in solar cars and part of light cars,
have recently come to be mounted in an exceedingly wide range from
small passenger cars to large passenger cars. In this field, the
number of cars equipped with the apparatus doubles year by
year.
[0004] Electric power steering apparatus for motor vehicles are
explained below in detail. The automotive power steering apparatus
mainly used at present are of the hydraulic type. In the case of
the hydraulic type, however, it is necessary to directly take a
measure not to cause an environmental problem because a hydraulic
fluid (power steering fluid) is used. In addition, since the
hydraulic pump for generating a hydraulic pressure is operated by a
power from the engine and is always operated (even when the
steering wheel is not manipulated), use of the hydraulic apparatus
is accompanied by an engine power loss. This is a cause of
impairment of fuel efficiency.
[0005] On the other hand, in the case of an electric power steering
apparatus, an electric motor is used as a power source for power
assist. It is sufficient to drive the electric motor with a
controller only when power assist is necessary. Furthermore, since
the electric motor is driven by the electricity generated during
car running, the engine power loss is extremely small and the
effect of saving fuel cost is high. Consequently, the amount of
energy consumption also can be far smaller than in the hydraulic
type.
[0006] However, the electric power steering apparatus presently in
use are still inferior in output power to hydraulic power steering
apparatus. It is hence important to improve the ability of the
electric motor and to minimize the friction of individual
constituent parts and thereby minimize the burden to be imposed on
the motor.
[0007] Furthermore, in electric power steering apparatus,
low-temperature starting is regarded as important especially in
cold districts. The reasons for this are as follows. In the case of
hydraulic power steering apparatus, engine warming-up has an effect
that the heat source of the hydraulic pump, which is directly
connected to the engine, warms up each part of the steering
apparatus using the hydraulic oil as a heat medium. Because of
this, a lubricant having general low-temperature characteristics
has sufficed for the hydraulic steering apparatus. In the case of
electric power steering apparatus, however, the steering apparatus
is not readily warmed up because of the absence of a direct heat
source from the engine.
[0008] Consequently, it is essential that the greases to be used in
parts for the apparatus should be ones which attain stable friction
torque properties at low temperatures.
[0009] Motor vehicles are used all over the world. Electric power
steering apparatus also are hence designed and produced while
taking account of common use in environments ranging from an
extremely cold environment of about -40.degree. C. to a red-heat
environment of 100.degree. C. or higher [(outside air
temperature)+(radiant heat in engine room)+(radiant heat from road
surface)]. There is a desire for a grease which has stable
low-torque characteristics in a wide temperature range from a low
temperature to a high temperature, does not suffer the oil film
deficiency caused by a viscosity decrease at high temperatures, and
has a long life corresponding to the life of the vehicle.
[0010] Furthermore, there recently are rapidly increasing trends
toward the active incorporation of subtle and delicate behaviors
according to natural laws, such as the vibrations, fluctuations,
and fuzzy movements which are sensed by men, into automotive parts
and toward an offer of more comfortable and stable handling and a
more comfortable interior space. New techniques have rapidly come
to be desired.
[0011] Electric power steering apparatus are steering apparatus
which will be mainly used in the future in place of the hydraulic
steering apparatus and, hence, are progressing in an innovational
manner. Under these circumstances, the introduction of
unconventional new ideas and techniques is remarkable. One of
important subjects is to evolve subtle feelings sensed by men, such
as those described above, in a more comfortable direction. This
tendency is exceedingly strong especially in high-class passenger
cars.
[0012] A specific example of such feelings related to the invention
is as follows. In the case where a car is running straight on a
straight highway, the steering apparatus is in a load-free state
and, hence, the electric motor which operates steering is in the
state of being stopped and giving no assist. In this situation, the
driver's hands gripping the steering wheel are not in a completely
stationary state. The tires are moving little by little because of
road surface undulations, etc. and the steering wheel, which
receives such subtle vibrations and movements, subtly swings in the
leftward/rightward directions. The driver's hands gripping the
steering wheel sense these subtle movements, and the driver is
continuously in the state of driving the car while manipulating the
steering wheel little by little in the leftward/rightward
directions.
[0013] In this period of subtle manipulation, electric assist is
not given. Consequently, the subtle feelings given to the driver's
hands are attributable to the direct transmission of frictions
generated at individual parts constituting the steering system. (In
the case where the steering wheel is manipulated and electric
assist is given, the frictions generated at the individual parts
are countervailed by the power of electric assist and are hence
difficult to be transmitted to the driver's hands.)
[0014] Examples of the frictions thus occurring at individual parts
constituting the steering mechanism include frictions at the
following parts which are shown in the FIG. 2 given in patent
document 1: the sliding parts of the sealed bearings 33 and 34; the
contact part between the teeth of the rack shaft 15 and the teeth
of the pinion shaft 22; and the sliding part between either the
ball screw groove 15b or the ball screw groove 38A and the ball
bearing 39 interposed between these grooves.
[0015] When such frictions at the sliding parts of individual parts
fluctuate irregularly, there is a high possibility that these
fluctuations are directly transmitted to the driver's hands
grasping the steering wheel and are sensed as an unpleasant
feeling. Although the irregular fluctuations of friction occurring
here practically exert no influence on steering wheel manipulation,
the subtle feeling given to men is a serious problem in cars of the
kind for which exceedingly high quality and ultimate
comfortableness are pursued, such as high-class passenger cars.
[0016] In the case of ball screw devices having the same mechanism
as the ball screw structure in the electric power steering
apparatus described above, the ball screw devices are generally
employed mainly in machine tools. However, the mechanism of the
rolling/sliding of balls is almost the same and use of general
lubricating greases is apt to result in friction fluctuations at
the sliding part between the balls and a screw part. In case where
irregular friction fluctuations have occurred, processing accuracy
decreases and higher quality cannot be imparted to the work.
[0017] There are many documents on rolling devices, electric power
steering apparatus, and lubricants for the apparatus. However,
there is no document on a rolling device which discloses an
electric power steering apparatus in which the technique concerning
a grease composition according to that patent document is used to
diminish the subtle unpleasant feeling given to men and thereby
enable more comfortable manipulation and which further discloses a
technique for inhibiting the irregular friction fluctuations.
[0018] For example, documents on rolling devices concerning
lubrication include patent documents 2 to 5.
[0019] Patent document 2 discloses a technique for prolonging the
lubrication life of a high-load ball screw to be incorporated into
the injection drive shaft or mold clamping mechanism driving part
of an injection molding machine of the electrically operated type
or into an electric press, bender, or the like. In this technique,
a grease containing a urea compound, an organonickel compound, and
an organomolybdenum compound and having a 100,000-time worked
consistency as measured in accordance with JIS K-2220 of 280 or
higher is enclosed to thereby inhibit the balls of the ball screw
from being marred and thereby attain life prolongation. Although
the urea compound shown in this patent document and part of the
additives shown therein closely resemble part of the grease
composition of the present invention, the grease composition
disclosed in that document basically differs from the grease
composition of the invention and utterly differs in the effect
obtained.
[0020] Patent document 3 discloses a grease for the ball screw in
the driving apparatus of an electrically operated injection molding
machine. This grease comprises a urea compound as a thickener and a
mineral oil or synthetic oil having a viscosity at 40.degree. C. of
300 mm.sup.2/s or higher as a base oil, and is characterized by
having a consistency of 300 or higher. The technique disclosed is
intended to prevent early damage. Although part of the urea
compound thickener shown in the patent document closely resembles
part of the grease composition of the invention, the grease
composition disclosed therein basically differs from the grease
composition of the invention and utterly differs in the effect
obtained.
[0021] Patent document 4 discloses a technique concerning a
lubricating grease for lubricating the lubrication passageways of a
ball screw in which a screw groove has been formed in each of the
outer peripheral surface of the screw shaft and the inner
peripheral surface of the nut. This lubricating grease has a
lightness of 1 or higher and has a composition obtained by
incorporating additives including an organomolybdenum compound into
a lubricating grease prepared by thickening a base oil having a
viscosity at 100.degree. C. of 3.0-7.5 mm.sup.2/s with a urea
compound thickener. The effects of this technique are as follows.
Since the lubricating grease enclosed has a lightness of 1 or
higher, the lubricating grease changes to black upon deterioration
and decreases in lightness to below 1. Consequently, the timing of
greasing can be easily noticed. Furthermore, the use of a urea
compound thickener in combination with a base oil having a
viscosity at 100.degree. C. of 3.0-7.5 mm.sup.2/s enables the
lubricating grease to be smoothly supplied to the friction surfaces
to thereby improve the durability of the ball screw. Although part
of the urea compound thickener and part of the additives in the
grease composition disclosed in the patent document closely
resemble part of the grease composition of the invention, the
grease composition disclosed therein basically differs from the
grease composition of the invention and utterly differs in the
effect obtained.
[0022] Patent document 5 discloses a technique concerning a method
of lubricating a ball screw with a lubricating oil which comprises
a poly(.alpha.-olefin) oil, an alkylbenzene oil, and an
ethylene/propylene oligomer as a base oil and contains an
extreme-pressure agent such as an ash-free thiocarbamate compound,
hydroxymolybdenum dithiocarbamate sulfide, or the like and which
has a dynamic viscosity at 40.degree. C. of 10-460 mm.sup.2/s. This
technique is for preventing the abnormal wearing and seizing of a
ball screw by diminishing frictional heat in the sliding part or
removing wearing dust and foreign matters. The lubricant disclosed
in the patent document is a lubricating oil and intrinsically
differs from the grease composition of the invention. This
lubricating oil utterly differs in the effect thereof to the
technique of the invention.
[0023] There also are patent documents 6 to 11 as documents on an
electric power steering apparatus and a lubricant for the
apparatus.
[0024] Patent document 6 describes a grease composition for
automotive steering mechanisms which comprises (a) thickener, (b) a
base oil having a pour point of -40.degree. C. or lower, (c) an
organomolybdenum compound, (d) melamine cyanurate, (e)
polytetrafluoroethylene, and (f) molybdenum disulfide. There is a
statement therein to the effect that the grease composition for
automotive steering mechanisms exhibits adequate lubricity in a
gear toothing part especially in a rack-and-pinion part, the hypoid
gears of a pinion-assisted electric power steering mechanism, or
the like. However, the makeup of this grease composition utterly
differs from that of the grease composition of the invention.
[0025] Patent document 7 describes an electric power steering
apparatus employing as a lubricant a grease which has improved
long-lasting high-temperature lubricity while retaining intact
low-starting-torque characteristics at low temperatures. There is a
statement therein to the effect that the grease comprises a base
oil which is a synthetic hydrocarbon oil, a thickener selected from
lithium composite soaps and urea compounds, and a lubricity
improver selected from solid lubricants and oily substances. There
also is a statement therein to the effect that the electric power
steering apparatus is a power steering apparatus which has an
electric motor for generating steering assist power and a speed
reducer serving to reduce a rotation speed by means of a gear
mechanism connected to the rotation shaft of the motor and in which
at least one of the reduction gears of the gear mechanism is made
of a synthetic resin and this synthetic-resin gear is lubricated
with a grease. With respect to the grease composition, a claim
includes the term urea compound thickener. This grease composition
is coincident in this point only. However, details of the makeup of
the composition and an effect are not shown at all.
[0026] Patent document 8 describes a lubricating grease composition
which comprises a base oil and a thickener and in which a
fluororesin powder has been added to the base oil. There is a
statement therein to the effect that this grease composition is
used in e.g., the speed reducer of an electric power steering
apparatus.
[0027] Patent document 9 describes a lubricating grease composition
which comprises a base oil and a thickener and in which the
thickener comprises a combination of lithium stearate and lithium
hydroxystearate. There is a statement therein to the effect that
this grease composition is used in, e.g., the speed reducer of an
electric power steering apparatus.
[0028] That speed reducer of an electric power steering apparatus
which is described in patent document 8 and patent document 9 and
the specific part therein to be lubricated are, for example, a
speed reducer employing a worm wheel made of a synthetic resin,
such as a polyamide resin. There is a statement therein to the
effect that the role of a lubricating grease composition
contributing to friction reduction in the part to be lubricated in
the sliding part (friction surfaces) between the worm wheel made of
a synthetic resin and the worm shaft made of a metal is important.
However, this grease composition utterly differs from the grease
composition of the invention.
[0029] Patent document 10 describes a grease composition for resin
lubrication obtained by incorporating montan wax into a grease
comprising a thickener and a base oil. There is a statement therein
to the effect that the grease composition is for the speed reducer
of an electric power steering apparatus and the specific part to be
lubricated therewith is the speed reduction mechanism part
including a worm wheel gear made of a resin (polyamide) and a worm
gear made of steel. With respect to the grease composition, a claim
includes the term urea compound thickener. This grease composition
is coincident in this point only. However, this grease composition
utterly differs from the grease composition of the invention as
apparent from the facts that it contains montan wax as an essential
ingredient and that it is a grease composition for resin
lubrication.
[0030] Patent document 11 describes a lubricating grease
composition which comprises a base oil and a thickener and in which
a polyethylene oxide wax has been incorporated. There is a
statement therein to the effect that the grease composition is used
for, e.g., the speed reducer of an electric power steering
apparatus. The speed reducer of an electric power steering
apparatus is a speed reducer comprising a worm wheel made of a
synthetic resin and a worm shaft made of a metal, and the specific
part to be lubricated with the grease composition is the friction
surfaces of this speed reducer. However, this grease composition
utterly differs from the grease composition of the invention.
[0031] Patent document 12 discloses an electric power steering
apparatus in which the assist power of an electric motor is
transmitted to the steering mechanism of a motor vehicle through a
speed reduction gear mechanism. This electric power steering
apparatus is characterized in that the follower gear of the speed
reduction gear mechanism comprises a metallic core pipe and,
integrally formed on the outer periphery of the pipe, a resinous
part made of a resin composition and having gear teeth formed in
the periphery thereof and that the speed reduction gear mechanism
is lubricated with a grease composition which contains a wax having
a polar group introduced in the molecular structure thereof and
employs a thickener such as a diurea compound. In this apparatus,
the sliding lubrication of the interface between the resinous
member and a metallic member, which is the sliding part of the
speed reduction gear mechanism, is satisfactorily maintained over
long to thereby give an excellent steering feeling. Although part
of the diurea compound and part of the additive ingredients in the
grease composition disclosed in the patent document closely
resemble part of the grease composition of the invention, the
grease composition disclosed therein basically differs from the
grease composition of the invention. In addition, the electric
power steering apparatus utterly differs in constituent parts and
friction mechanism.
[0032] The electric power steering apparatus according to the
invention is an apparatus shown in the accompanying drawings in
patent document 1, i.e., a rack-assisted electric power steering
apparatus constituted of a ball screw mechanism 37 and rolling
bearings 33 and 34. This apparatus gives power assist in the shaft
directions based on the ball screw connected to the rack shaft.
Because this ball screw mechanism closely resembles the ball screw
mechanisms mounted in machine tools and the like, the lithium
grease frequently employed in these machine tools has been used as
the lubricating grease for that part.
[0033] The rack-assisted electric power steering apparatus
disclosed in patent document 1 is one in which the electric motor
has been disposed coaxially with the rack shaft. However, there
also are the type in which the electric motor has been disposed not
coaxially but parallel with the rack shaft (e.g., patent document
13) and the type in which the electric motor has been disposed so
that the shaft axis thereof intersects that of the rack shaft
(e.g., patent document 14). In these types, the electric motor is
connected to the ball screw mechanism (ball screw nut) through a
transmission device such as a gear mechanism or a belt.
[0034] However, as the electric power steering apparatus improve in
assist power, durability, and stable low-torque characteristics and
remarkably progress in other various performances with the
increasing desire for the mounting of the apparatus in motor
vehicles, the lubricating greases according to prior-art techniques
have become unable to enable the steering apparatus to sufficiently
exhibit stable steering characteristics at temperatures ranging
from a high to a low temperature and have a satisfactory durability
life. In addition, the prior-art lubricating greases have had a
problem concerning low-torque characteristics in lower-temperature
environments.
[0035] The "rolling bearing" employed in each of the electric power
steering apparatus described above mainly is a deep-groove ball
bearing having one or more grooves (see, for example, patent
document 15).
[0036] The "electric power steering apparatus" according to the
invention should not be construed as being limited to those
described above, and can be one such as those described in, e.g.,
patent document 13 and patent document 14. In the former case, the
ball screw mechanism 2 and each ball bearing described in the
document correspond to the "rolling bearing" and, in particular,
the bearings 8 and 10 which movably support the nut part 2a
correspond to the "ball bearing which utilizes the rolling of
balls". In the latter case, the ball screw mechanism 9 (19, 29) and
each ball bearing described in the document correspond to the
"rolling bearing" and, in particular, the bearing 10 (20) which
movably supports the nut part 9a (19a, 29a) corresponds to the
"ball bearing which utilizes the rolling of balls".
[0037] The rolling device is a rolling device utilizing the rolling
of balls, and rolling devices having any ball screw mechanism are
included therein.
[0038] The present inventors made thorough investigations on
constituent parts of the electric power steering apparatus
described especially in patent document 1 as to which parts are apt
to give a subtle unpleasant feeling to men. As a result, they found
that constituent parts suffering rolling/sliding friction, such as
the sealed bearings 33 and 34 and the sliding part between either
the ball screw groove 15b or the ball screw groove 38A and the ball
bearing 39 interposed between these grooves, are apt to undergo
irregular friction fluctuations. It was further found that the
degree of these irregular friction fluctuations is considerably
influenced by the difference of the lubricating grease. The
inventors made further intensive investigations. As a result, they
have found that the lubricating grease composition of the invention
can greatly reduce the irregular friction fluctuations occurring at
those constituent parts and is an extremely stable lubricating
grease composition which does not give an unpleasant feeling to the
driver at all. Based on these findings, the inventors have
succeeded in providing a rolling device and an electric power
steering apparatus each containing the lubricating composition.
[0039] Patent Document 1: JP-A-2003-335249 [0040] Patent Document
2: JP-A-2000-303089 [0041] Patent Document 3: JP-A-2001-49274
[0042] Patent Document 4: JP-A-2001-304307 [0043] Patent Document
5: JP-A-2002-340132 [0044] Patent Document 6: JP-A-2001-64665
[0045] Patent Document 7: JP-A-2002-308125 [0046] Patent Document
8: JP-A-2002-363589 [0047] Patent Document 9: JP-A-2002-363590
[0048] Patent Document 10: JP-A-2002-371290 [0049] Patent Document
11: JP-A-2003-3185 [0050] Patent Document 12: JP-A-2004-301268
[0051] Patent Document 13: JP-A-2004-114972 [0052] Patent Document
14: JP-A-2004-122858 [0053] Patent Document 15:
JP-A-2004-144118
DISCLOSURE OF THE INVENTION
Problems that the Invention is to Solve
[0054] The invention relates to a rolling device employing a
lubricating grease composition which considerably diminishes
irregular friction fluctuations occurring on rolling/sliding
friction surfaces, shows stable low-torque characteristics in a
wide temperature range, can retain a sufficient oil film even at
high temperatures, has a long life, and comprises a novel
thickener, a specific base oil, and additive ingredients. The
invention further relates to an electric power steering
apparatus.
Means for Solving the Problems
[0055] The present inventors made thorough investigations on the
sliding parts of rolling devices in electric power steering
apparatus. As a result, they found that constituent parts which
suffer rolling/sliding friction come into the state of being apt to
undergo irregular friction fluctuations depending on the difference
of the lubricating grease. The inventors made further intensive
investigations. As a result, it has been found that when a
lubricating grease composition comprising a novel thickener, a
specific base oil, and additive ingredients is applied to a rolling
device, then a rolling device having a long life is obtained which
can be considerably reduced in irregular friction fluctuations,
shows stable low-torque characteristics in a wide temperature
range, and can retain a sufficient oil film even at high
temperatures. Furthermore, it has been found that a
high-performance electric power steering apparatus is obtained by
employing this rolling device. The invention has been thus
achieved.
[0056] Namely, the invention, in a first aspect, relates to a
rolling device characterized by employing a lubricating grease
composition comprising:
[0057] a thickener which is a mixed thickener comprising a mixture
of:
[0058] (a) one or more diurea compounds which are one or more
aliphatic diurea compounds having an average molecular weight of
500-750 in which 10-70% by mole of the linear hydrocarbon groups
are unsaturated components, the aliphatic diurea compounds having
been produced using as a raw material one or more primary amines
having a total amine value in the range of 200-400,
[0059] (b) at least one fatty acid metal salt, and
[0060] (c) at least one amide compound selected from the group
consisting of aliphatic amides and aliphatic bisamides represented
by formulae (I) and (II):
R.sub.1CONH.sub.2 (I)
R.sub.1CONHR.sub.2NHCOR.sub.1 (II)
[0061] (wherein R.sub.1 represents a saturated or unsaturated alkyl
group having 15-17 carbon atoms and R.sub.2 represents a methylene
group or an ethylene group),
the proportions of (a) and of (b) and (c) satisfying the
relationship:
a/(b+c)=0.2-10
provided that
[0062] (1) the proportion of ingredient (a) is 1-10,
[0063] (2) the proportion of ingredient (b) is 0.5-2.5, and
[0064] (3) the proportion of ingredient (c) is 0.5-2.5, the amount
of the thickener being 2-30% by weight based on the whole
composition;
[0065] (d) a base oil which is a lube base oil having a pour point
of -25.degree. C. or lower and comprising a synthetic hydrocarbon
oil as the main component; and
[0066] (e) an additive which is a mixture comprising an
organomolybdenum complex, an organozinc compound of a
dithiocarbamic acid, and an organozinc compound of a
dithiophosphoric acid, the amount of the additive being 1-7% by
weight, based on the whole composition.
[0067] The invention, in a second aspect, relates to the rolling
device of claim 1 characterized in that the rolling device is a
ball screw mechanism.
[0068] The invention, in a third aspect, relates to the rolling
device of claim 1 characterized in that the rolling device is a
rolling bearing.
[0069] The invention, in a fourth aspect, relates to the rolling
device of claim 3 characterized in that the rolling bearing is a
ball bearing which utilizes the rolling of balls.
[0070] The invention, in a fifth aspect, relates to an electric
power steering apparatus characterized by employing the rolling
device of any one of claims 1 to 4.
[0071] In the invention, in case where the average molecular weight
of the thickener (a) is lower than 500 and in case where the
average molecular weight thereof exceeds 750, optimal grease
intervention is not obtained in the device and stable torque
characteristics are not sufficiently obtained.
[0072] In case where the proportion of unsaturated components in
the linear hydrocarbon groups is lower than 10% by mole, a proper
oiliness effect is not obtained in the device and stable torque
characteristics are not sufficiently obtained. In case where the
proportion thereof exceeds 70% by mole, it is difficult to secure
sufficient heat resistance in the device and life shortening is
expected. Furthermore, in case where the total amine value of the
primary amines is outside the range of 200-400, it is difficult to
obtain optimal grease intervention, a proper oiliness effect, and
sufficient heat resistance in the device. In this case, it is
expected that stable torque characteristics are not sufficiently
obtained and the life is shortened.
[0073] The fatty acid metal salt as ingredient (b) is, for example,
a salt formed by reacting a linear, saturated or unsaturated,
aliphatic monocarboxylic acid having 6-24 carbon atoms (the acid
may contain one hydroxyl group), such as lauric acid, myristic
acid, palmitic acid, stearic acid, 12-hydroxystearic acid, arachic
acid, behenic acid, lignoceric acid, oleic acid, linoleic acid,
linolenic acid, or ricinoleic acid, with a metal such as lithium,
sodium, magnesium, aluminum, calcium, zinc, or barium. Especially
preferred are fatty acid metal salts which are the lithium,
magnesium, aluminum, calcium, or zinc salts of saturated or
unsaturated aliphatic monocarboxylic acids having 12-18 carbon
atoms.
[0074] The at least one amide compound as ingredient (c) is a
compound obtained by reacting a fatty acid with an amine, and
comprises a mixture of at least one amide compound selected from
the group consisting of aliphatic amides and aliphatic bisamides
represented by formulae (I) and (II):
R.sub.1CONH.sub.2 (I)
R.sub.1CONHR.sub.2NHCOR.sub.1 (II)
[0075] (wherein R.sub.1 represents a saturated or unsaturated alkyl
group having 15-17 carbon atoms and R.sub.2 represents a methylene
group or an ethylene group). Examples thereof include
N,N'-ethylenebisstearamide, N,N'-methylenebisstearamide,
stearamide, and oleamide.
[0076] Examples of the synthetic hydrocarbon oil to be used as or
in ingredient (d) in the invention include poly .alpha.-olefins,
polybutenes, and ethylene/.alpha.-olefin co-oligomers. These
hydrocarbon oils exhibit an optimal effect in the device when they
each have a pour point of -25.degree. C. or lower. In case where a
lubricating oil having a pour point higher than -25.degree. C. is
used in a grease, this grease itself has enhanced viscoelasticity
and given low-temperature torque characteristics cannot be obtained
in the device.
[0077] Although the base oil for use in the grease composition
comprises any of those synthetic hydrocarbon oils as the main
component, the synthetic hydrocarbon oils have the following and
other excellent features. These oils have a low pour point and can
retain flowability even at low temperatures. They have satisfactory
oxidative stability and excellent viscosity/temperature
characteristics (high VI). Because of this, even at high
temperatures, the oils decrease little in viscosity and can retain
a stable oil film. In addition, those oils do not exert adverse
influences, e.g., swelling, on members made of synthetic rubbers,
synthetic resins, etc.
[0078] A mineral oil, synthetic ester oil, synthetic polyglycol
oil, synthetic silicone oil, synthetic fluorochemical oil, or the
like can be used as part of the base oil for use in the grease
composition in the device. However, these oils are unsuitable for
use as a main component. In case where a mineral oil is used as a
major component of the base oil, this base oil is insufficient in
low-temperature characteristics and heat resistance. In case where
a synthetic ester oil is used, there is a fear that it may exert
adverse influences on synthetic rubbers and synthetic resins, such
as swelling the rubbers or resins or reducing the hardness thereof.
Synthetic polyglycol oils are inferior in heat resistance or
lubricity to synthetic hydrocarbon oils.
[0079] Furthermore, synthetic silicone oils are less apt to give
satisfactory lubricity although excellent in heat resistance.
Synthetic fluorochemical oils have a poor affinity for the
thickener and are highly expensive although exceedingly excellent
in heat resistance. The term main component used with regard to (d)
means that the synthetic hydrocarbon oil having a pour point of
-25.degree. C. or lower accounts for 80% by weight or more of the
lubricating oil.
[0080] The organomolybdenum complex as a component of ingredient
(e) in the invention means the organomolybdenum complex described
in JP-B-5-66435. Examples of the organozinc compound of a
dithiocarbamic acid include zinc dithiocarbamates such as zinc
diethyldithiocarbamate sulfide, zinc dipropyldithiocarbamate
sulfide, zinc dibutyldithiocarbamate sulfide, zinc
dipentyldithiocarbamate sulfide, zinc dihexyldithiocarbamate
sulfide, zinc didecyldithiocarbamate sulfide, zinc
diisobutyldithiocarbamate sulfide, zinc
di(2-ethylhexyl)dithiocarbamate sulfide, zinc diamyldithiocarbamate
sulfide, zinc dilauryldithiocarbamate sulfide, zinc
distearyldithiocarbamate sulfide, zinc diphenyldithiocarbamate
sulfide, zinc ditolyldithiocarbamate sulfide, zinc
dixylyldithiocarbamate sulfide, zinc diethylphenyldithiocarbamate
sulfide, zinc dipropylphenyldithiocarbamate sulfide, zinc
dibutylphenyldithiocarbamate sulfide, zinc
dipentylphenyldithiocarbamate sulfide, zinc
dihexylphenyldithiocarbamate sulfide, zinc
diheptylphenyldithiocarbamate sulfide, zinc
dioctylphenyldithiocarbamate sulfide, zinc
dinonylphenyldithiocarbamate sulfide, zinc
didecylphenyldithiocarbamate sulfide, zinc
didodecylphenyldithiocarbamate sulfide, zinc
ditetradecylphenyldithiocarbamate sulfide, and zinc
dihexadecylphenyldithiocarbamate sulfide. Examples of the
organozinc compound of dithiophosphoric acid include zinc
diethyldithiophosphate sulfide, zinc dipropyldithiophosphate
sulfide, zinc dibutyldithiophosphate sulfide, zinc
dipentyldithiophosphate sulfide, zinc dihexyldithiophosphate
sulfide, zinc didecyldithiophosphate sulfide, zinc
diisobutyldithiophosphate sulfide, zinc
di(2-ethylhexyl)dithiophosphate sulfide, zinc diamyldithiophosphate
sulfide, zinc dilauryldithiophosphate sulfide, zinc
distearyldithiophosphate sulfide, zinc diphenyldithiophosphate
sulfide, zinc ditolyldithiophosphate sulfide, zinc
dixylyldithiophosphate sulfide, zinc diethylphenyldithiophosphate
sulfide, zinc dipropylphenyldithiophosphate sulfide, zinc
dibutylphenyldithiophosphate sulfide, zinc
dipentylphenyldithiophosphate sulfide, zinc
dihexylphenyldithiophosphate sulfide, zinc
diheptylphenyldithiophosphate sulfide, zinc
dioctylphenyldithiophosphate sulfide, zinc
dinonylphenyldithiophosphate sulfide, zinc
didecylphenyldithiophosphate sulfide, zinc
didodecylphenyldithiophosphate sulfide, zinc
ditetradecylphenyldithiophosphate sulfide, and zinc
dihexadecylphenyldithiophosphate sulfide.
[0081] In the invention, the proportions of (a) and of (b) and (c)
satisfy the relationship:
a/(b+c)=0.2-10
provided that (1) the proportion of ingredient (a) is 1-10, (2) the
proportion of ingredient (b) is 0.5-2.5, and (3) the proportion of
ingredient (c) is 0.5-2.5. In case where a/(b+c) is smaller than
0.2, the proportion of the diurea ingredient is small and desired
heat resistance is not obtained. In case where a/(b+c) exceeds 10,
the effect of diminishing friction fluctuations is not sufficiently
obtained in the device.
[0082] In case where the proportion of ingredient (a) is smaller
than 1.0, which correlates to the relationship a/(b+c), the
proportion of the diurea ingredient is small, resulting in
insufficiency of desired heat resistance. In case where the
proportion thereof exceeds 10, the effect of diminishing friction
fluctuations is not sufficiently obtained in the device.
[0083] Furthermore, in case where the proportions of ingredient (b)
and ingredient (c) each is smaller than 0.5, the effect of
diminishing friction fluctuations is not sufficiently obtained in
the device in this case also. When the proportions thereof exceed
2.5, the effect of diminishing friction fluctuations is not very
improved for such large amounts of the fatty acid metal salt and
amide compound. In this case, torque increases rather than
decreases, and insufficient heat resistance results because of the
small diurea ingredient proportion.
[0084] In case where the amount of the whole thickener mixed is
smaller than 2% by weight, the grease is too soft and there is a
fear about leakage, etc. In case where the amount thereof is larger
than 30% by weight, the grease is too hard and has increased flow
resistance, resulting in a considerable increase in torque and an
increased cost.
[0085] In case where the proportion of the synthetic hydrocarbon
oil ingredient in ingredient (d) is smaller than 80% by weight, a
stable low torque is not obtained throughout the range from
ordinary temperature to a low temperature.
[0086] The content of ingredient (e) is 1-7% by weight, preferably
1.5-6% by weight, more preferably 2-5% by weight, based on the
whole composition.
[0087] In case where the content of ingredient (e) is lower than 1%
by weight based on the whole composition, the device does not have
a given durability life. When the content thereof is higher than 7%
by weight, no remarkable effect is obtained, resulting only in an
increased cost.
ADVANTAGES OF THE INVENTION
[0088] The invention has succeeded in providing a rolling device
and an electric power steering apparatus which each employ as a
lubricant a long-life lubricating grease composition which
considerably diminishes irregular friction fluctuations occurring
on rolling/sliding friction surfaces, shows stable low-torque
characteristics in a wide temperature range, can retain a
sufficient oil film even at high temperatures, and contains a novel
thickener.
EXAMPLES
[0089] The invention will be explained below in detail by reference
to Examples and Comparative Examples, but the invention should not
be construed as being limited by these Examples in any way.
Examples 1 to 4
[0090] Grease samples were produced according to the formulations
shown in Table 1.
[0091] The method used is as follows. The base oil and the
diisocyanate were charged into a closed type grease sample
production apparatus. The contents were heated to 60.degree. C.
with stirring, and a raw material prepared by mixing the amines
with the base oil and dissolving these was added to the contents
through a hopper and reacted therewith. The reaction mixture was
heated to 170.degree. C. with stirring and held for 30 minutes to
complete the reaction. Thereafter, the reaction mixture was rapidly
cooled. During this cooling step, the fatty acid metal salt and the
amide compound were added to the reaction mixture in the
proportions shown in Table 1 and mixed therewith by stirring. This
mixture was cooled to 80.degree. C. Furthermore, the additives were
added thereto in the proportions shown in Table 1. Finally,
octyldiphenylamine was added as an antioxidant to each grease
sample in an amount of 1.0% based on all ingredients other than
this antioxidant. The resultant mixture was allowed to cool to
about 60.degree. C. and then treated with a homogenizer. Thus,
greases were obtained.
Comparative Examples 1 to 4
[0092] Grease samples were produced according to the formulations
shown in Table 2.
[0093] The method used is as follows. The base oil and the
diisocyanate were charged into a closed type grease sample
production apparatus. The contents were heated to 60.degree. C.
with stirring, and a raw material prepared by mixing the amines
with the base oil and dissolving these was added to the contents
through a hopper and reacted therewith. The reaction mixture was
heated to 170.degree. C. with stirring and held for 30 minutes to
complete the reaction. Thereafter, the reaction mixture was rapidly
cooled. During this cooling step, the fatty acid metal salt and/or
the amide compound was added to the reaction mixture in the
proportions shown in Table 2 and mixed therewith stirring. This
mixture was cooled to 80.degree. C. Furthermore, the additives were
added thereto in the proportions shown in Table 2. Finally,
octyldiphenylamine was added as an antioxidant to each grease
sample in an amount of 1.0% based on all ingredients other than
this antioxidant. The resultant mixture was allowed to cool to
about 60.degree. C. and then treated with a homogenizer. Thus,
greases were obtained.
[0094] The diisocyanate used as a raw material for the diurea
compounds (a) shown in Tables 1 and 2 is the compound of the
following chemical structure, which has a molecular weight of
250.
[Chemical Structure-1]
##STR00001##
[0096] Amine A is a linear primary amine consisting mainly of (at
least 90%) a saturated alkyl group with 8 carbon atoms and having
an average molecular weight of 130 (industrial caprylamine).
[0097] Amine B is a linear primary amine consisting mainly of (at
least 90%) a saturated alkyl group with 18 carbon atoms and having
an average molecular weight of 270 (industrial stearylamine).
[0098] Amine C is a linear primary amine which contains about 50%
unsaturated alkyl group with 18 carbon atoms and further contains
saturated or unsaturated alkyl groups with 18-14 carbon atoms and
which has an average molecular weight of 255 (industrial beef
tallow amine).
[0099] Amine D is a linear primary amine consisting mainly of (at
least 70%) an unsaturated alkyl group with 18 carbon atoms and
having an average molecular weight of 260 (industrial
oleylamine).
[0100] The fatty acid metal salts (b) are as follows.
[0101] Fatty acid metal salt A is the lithium salt of
12-hydroxystearic acid.
[0102] Fatty acid metal salt B is the lithium salt of stearic
acid.
[0103] Fatty acid metal salt C is the calcium salt of stearic
acid.
[0104] Fatty acid metal salt D is the aluminum salt of stearic
acid.
[0105] The amide compounds (c) are as follows.
[0106] Amide A is N,N'-ethylenebisstearamide.
[0107] Amide B is stearamide.
[0108] The mineral oil shown in the Examples and Comparative
Examples is one having a dynamic viscosity at 40.degree. C. of
101.5 mm.sup.2/s and a pour point of -15.degree. C. Synthetic
hydrocarbon oil A (CAS No. 68037-01-4) is one having a dynamic
viscosity at 40.degree. C. of 14.94 mm.sup.2/s and a pour point of
-67.7.degree. C. Moreover, synthetic hydrocarbon oil B (CAS No.
68037-01-4) is one having a dynamic viscosity at 40.degree. C. of
396.2 mm.sup.2/s and a pour point of -36.degree. C.
[0109] Additive A is an organomolybdenum complex, which is the
oil-soluble organomolybdenum compound described in
JP-B-5-66435.
[0110] Additive B is primary Zn-DTP (primary zinc
dithiophosphate).
[0111] Additive C is Zn-DTC (zinc dithiocarbamate)
[0112] The Examples and Comparative Examples were evaluated for the
properties shown in the Tables according to the following test
methods. The tests were entrusted to K.K. SVC Tokyo.
[0113] Consistency: JIS K2220
[0114] Dropping point: JIS K2220
[0115] Oil separation: JIS K2220, method B; conditions, 100.degree.
C./24 hours.
[0116] The following were found from the results of these
experiments.
(1) The electric power steering apparatus employing the lubricating
greases of the invention showed stable torque characteristics in
which the irregular friction fluctuations occurring on
rolling/sliding friction surfaces had been considerably diminished.
(2) The electric power steering apparatus employing the lubricating
greases of the invention showed stable low-torque characteristics
in a wide temperature range. (3) The electric power steering
apparatus employing the lubricating greases of the invention have
an exceedingly long life even at high temperatures.
[0117] Examples of the electric power steering apparatus to which
the invention is to be applied include that described in
JP-A-2003-335249. The constitution of this apparatus is as
described in the document. As shown in the FIG. 2 of that document,
the apparatus comprises, as major components, a rack shaft 15
connected to wheels and an electric motor 30 disposed so as to
encircle the rack shaft 15, the rack shaft 15 having a ball screw
groove 15B formed in the peripheral surface thereof. The apparatus
further has a ball screw mechanism 37 comprising a ball screw nut
38 attached to one end of the motor shaft 32 of the electric motor
30. In this constitution, the ball screw mechanism 37 and bearings
33 and 34, which support both ends of the motor shaft 32, are
rolling devices. Of these, the bearings 33 and 34 are rolling
bearings, more particularly, ball bearings utilizing the rolling of
balls.
[0118] Samples were prepared by applying each of the greases of the
Examples and Comparative Examples as a lubricant to the rolling
devices of that electric power steering apparatus. These samples
were compared in performance by the following test methods.
1. Steering Wheel Sticking Test
[0119] When a driver manipulates a steering wheel (steers a car)
and any of the rolling devices, in particular, the ball screw
mechanism, of the steering apparatus comes to have a part where
friction fluctuates irregularly during the steering, then a region
appears where the steering torque abruptly increases during the
steering and the driver has a feeling of disorder such as steering
wheel sticking, i.e., an unpleasant feeling. In this test, each
electric power steering apparatus was hence used to conduct a
steering operation at ordinary temperature and steering torque
fluctuations in this operation were measured.
[0120] Specifically, the procedure is as follows. The steering
wheel was manipulated to operate the electric power steering
apparatus from the left-hand steering end to the right-hand
steering end and then back to the left-hand steering end, and the
load torque in this operation was measured. The torque values
measured were averaged (to determine the average torque). When
torque peaks having torque values larger than the average torque by
at least a given value were observed, the amount (protrudent torque
value) by which each peak protruded from the given value was
determined. The electric power steering apparatus samples were
evaluated (relative evaluation) by comparing these in the number of
peaks having a protrudent torque value (frequency of
protrudent-torque peaks). The greases of Examples 1 to 4 gave
results showing that they each had been stably low in the frequency
of protrudent-torque peaks as compared with Comparative Examples 2
and 3.
[0121] Consequently, when the greases of the Examples are used in a
rolling device, especially in a ball screw mechanism, the rolling
device in a machine tool or the like is expected to work stably and
smoothly. In the electric power steering apparatus employing this
rolling device, the driver who manipulates the steering wheel
(steers the car) has almost no feeling of disorder such as steering
wheel sticking and has a smooth and comfortable steering
feeling.
2. Low- to Ordinary-Temperature Performance Test
[0122] When the grease increases in viscosity in a low-temperature
region, the preload imposed on the rolling device, especially the
ball screw mechanism, increases and this may impair the steering
feeling (steering becomes heavy and, in particular, the so-called
"steering wheel return" becomes poor). In order to examine this
phenomenon, electric power steering apparatus having rolling
devices employing the greases of the Examples and Comparative
Examples were evaluated in the following manner. Each electric
power steering apparatus was regulated so as not to operate the
electric motor, i.e., so as to be in the state of giving no power
assist. Under these conditions, the steering torque required for
rotating the steering wheel by a given angle was measured. The
electric power steering apparatus were evaluated by comparing these
in the found value of the torque (preload).
[0123] As a result, the greases of Examples 1 to 4 showed
satisfactory performance in the range of from ordinary temperature
(25.degree. C.) to a low temperature (-20.degree. C.) as compared
with the grease of Comparative Example 4. Consequently, by applying
the greases of the Examples to a rolling device, especially to a
ball screw mechanism, a rolling device showing stable low-torque
characteristics is obtained. Furthermore, the electric power
steering apparatus employing this rolling device gives a
satisfactory steering feeling even when the apparatus has not been
warmed up just after engine starting or even in an extremely cold
district. In particular, the so-called "steering wheel return"
occurs smoothly. Thus, the superiority of the greases of the
Examples is exhibited.
3. Durability Test
[0124] When the grease decreases in viscosity in a high-temperature
region, there are cases where the rolling device comes into the
state in which the oil film to be formed from the grease has almost
disappeared (oil film deficiency). An oil film deficiency can occur
also as a result of grease deterioration or consumption due to
long-term use. This oil film deficiency not only inhibits smooth
lubrication in the rolling device to impair the steering feeling,
but also may cause seizing (sticking). When seizing occurs, this,
in the worst case, may result in breakage of the rolling device. In
order to examine this phenomenon, durability was examined in this
test under conditions similar to those in practical use.
[0125] Specifically, the procedure is as follows. Each lubricating
grease was charged into the rolling parts of the electric power
steering apparatus shown in JP-A-2003-335249. A normal-rotation
operation and reverse-rotation operation were repeatedly conducted
120,000 times while imposing a given load not lower than those in
practical use. The apparatus was examined for any change in working
state. Furthermore, after the test, the bearings and ball screw
mechanism were examined for surface state and damages/mars, and the
state of the grease was examined. The electric power steering
apparatus were evaluated by comparing these in durability.
[0126] As a result, it was found that satisfactory durability is
obtained by using the greases of Examples 1 to 4. Consequently,
when the greases of the invention are applied to a rolling device
and an electric power steering apparatus, the rolling device and
electric power steering apparatus obtained are less apt to suffer
an oil film deficiency, seizing, etc. and can retain high
reliability throughout long-term use.
TABLE-US-00001 TABLE 1 Example 1 2 3 4 (a) Diurea compounds
Diisocyanate (molar proportion) 1.0 1.0 1.0 1.0 Amine A (molar
proportion) 1.0 0.75 1.0 0.75 Amine B (molar proportion) 0.25 0.25
-- -- Amine C (molar proportion) -- 0.75 1.0 -- Amine D (molar
proportion) 0.75 0.25 -- 1.25 Average molecular weight of diurea
642.5 673.0 635.0 676.3 compounds (mole, MW) Proportion of
unsaturated components to 35.3 27.2 29.7 56.9 linear hydrocarbon
groups of diurea compounds (mol %) Total amine value of amines as
raw 289.1 265.7 291.6 264.6 material, mg-KOH/g Amount of ingredient
a (%) 5.0 5.0 8.0 11.0 (b) Fatty acid metal salts (%) Fatty acid
salt A 4.5 -- -- -- Fatty acid salt B -- 4.0 -- -- Fatty acid salt
C -- -- -- 2.0 Fatty acid salt D -- -- 3.5 -- (c) Amide compounds
(%) Amide A 3.5 2.0 -- 2.0 Amide B -- -- 3.5 -- Total amount of
ingredients b and c (%) 8.0 6.0 7.0 4.0 Proportion of ingredient a
1.43 2.5 4.57 8.25 Proportion of ingredient b 1.26 2.0 2.0 1.5
Proportion of ingredient c 1.0 1.0 2.0 1.5 Ratio of proportion of a
to proportion 0.625 0.833 1.143 2.75 of b and c; a/(b + c) Amount
of thickener (a + b + c) (%) 13.0 11.0 15.0 15.0 Mineral oil (%)
8.75 5.0 -- -- Synthetic hydrocarbon oil A (%) 75.0 74.5 76.0 81.0
Synthetic hydrocarbon oil B (%) -- 5.0 5.0 -- Additive A (%) 1.25
2.0 2.0 1.5 Additive B (%) 0.75 1.0 1.0 1.0 Additive C (%) 0.75 1.0
0.7 1.0 Additive D (%) 0.50 0.5 0.3 0.5 Total additive amount (%)
3.25 4.5 4.0 4.0 Consistency 311 322 295 255 Dropping point
(.degree. C.) 213 211 232 230 Oil separation (mass %) 0.35 0.36
0.32 0.29 Dynamic viscosity of base oil, 40.degree. C., mm.sup.2/s
15.69 18.39 18.00 14.94 Pour point of base oil (.degree. C) -56.9
-52.3 -61.5 -67.7 [Bench Test of Electric Power Steering] 1.
Steering wheel sticking test Frequency of protrudent-torque peaks
0.51 0.63 0.57 0.73 (ratio to the value for Comparative Example 3,
which was taken as 1.0) 2. Low- to ordinary-temperature performance
test (preload) (ratio to the preload for Comparative Example 3 at
-20.degree. C., which was taken as 1.0) -20.degree. C. 1.07 1.11
0.88 0.84 25.degree. C. 0.25 0.23 0.22 0.25 3. Durability test
normal normal normal normal in 120,000-time operation
TABLE-US-00002 TABLE 2 Comparative Example 1 2 3 4 (a) Diurea
compounds Diisocyanate (molar proportion) 1.0 1.0 1.0 1.0 Amine A
(molar proportion) 0.75 0.75 0.75 0.25 Amine B (molar proportion)
0.25 0.25 0.25 -- Amine C (molar proportion) 0.75 0.75 0.75 --
Amine D (molar proportion) 0.25 0.25 0.25 1.75 Average molecular
weight of diurea 673.0 673.0 673.0 743.7 compounds (mole, MW)
Proportion of unsaturated components 27.2 27.2 27.2 69.4 to linear
hydrocarbon groups of diurea compounds (mol %) Total amine value of
amines as raw 265.7 265.7 265.7 228.8 material, mg-KOH/g Amount of
ingredient a (%) 7.0 9.0 9.0 6.5 (b) Fatty acid metal salts (%)
Fatty acid salt A -- -- 0.25 -- Fatty acid salt B 6.0 -- -- --
Fatty acid salt C -- -- -- 3.5 Fatty acid salt D -- -- -- -- (c)
Amide compounds (%) Amide A -- -- 0.25 -- Amide B -- -- -- 3.5
Total amount of ingredients b and c 6.0 -- 0.5 7.0 (%) Proportion
of ingredient a 1.0 -- 10 3.71 Proportion of ingredient b 0.85 --
0.28 2.0 Proportion of ingredient c 0 -- 0.28 2.0 Ratio of
proportion of a to proportion 1.17 -- 18.0 2.67 of b and c; a/(b +
c) Amount of thickener (a + b + c) (%) 13.0 9.0 9.5 11.0 Mineral
oil (%) -- -- -- 35.3 Synthetic hydrocarbon oil A (%) 82.5 83.0
81.0 -- Synthetic hydrocarbon oil B (%) -- -- 5.0 52.9 Additive A
(%) 2.0 2.0 2.0 0.2 Additive B (%) 1.0 1.0 1.0 0.2 Additive C (%)
1.0 1.0 1.0 0.2 Additive D (%) 0.5 0.5 0.5 0.2 Total additive
amount (%) 4.5 4.5 4.5 0.8 Consistency 279 283 278 274 Dropping
point (.degree. C.) 228 226 230 189 Oil separation (mass %) 0.33
0.31 0.30 0.36 Dynamic viscosity of base oil, 40.degree. C., 14.94
14.94 17.81 263.1 mm.sup.2/s Pour point of base oil (.degree. C.)
-67.7 -67.7 -62.8 -21.0 [Bench Test of Electric Power Steering] 1.
Steering wheel sticking test Frequency of protrudent-torque peaks
0.84 1.22 1.0 0.80 (ratio to the value for Comparative Example 3,
which was taken as 1.0) 2. Low- to ordinary-temperature performance
test (preload) (ratio to the preload for Comparative Example 3 at
-20.degree. C., which was taken as 1.0) -20.degree. C. 0.94 0.82
1.0 2.36 25.degree. C. 0.28 0.22 0.24 0.35 3. Durability test
flaking normal normal flaking in 120,000-time operation occurred
occurred in 60,000 in 60,000 times times
[0127] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope
thereof.
[0128] This application is based on a Japanese patent application
filed on Apr. 28, 2005 (Application No. 2005-131702), the contents
thereof being herein incorporated by reference.
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